1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly to a semiconductor device having a magnetic tunnel resistance element as a resistance element.
2. Description of the Background Art
First, a constitution of a general-type filter will be discussed.
L-Shaped Primary Filter
FIGS. 39 and 40 show a low-pass filter (hereinafter, referred to as xe2x80x9cLPFxe2x80x9d) in which a resistor R and a capacitor C are connected to each other in the shape of L and a high-pass filter (hereinafter, referred to as xe2x80x9cHPFxe2x80x9d).
In FIG. 39, the resistor R is interposed between terminals T1 and T3 and the capacitor C is interposed between a wire connecting the terminals T2 and T4 and an end portion of the resistor R on the side of the terminal T3.
In FIG. 40, the capacitor C is interposed between the terminals T1 and T3 and the resistor R is interposed between the wire connecting the terminals T2 and T4 and an electrode of the capacitor C on the side of the terminal T3. Further, the terminals T1 and T2 serve as input terminals and the terminals T3 and T4 serve as output terminals.
Furthermore, as shown in FIG. 41, a filter in which impedances Z1 and Z 2 are connected to each other in the shape of L is referred to as an L-shaped primary filter, which includes the constitutions of FIGS. 39 and 40.
The characteristic of the filter is described by transfer function representing the ratio of an output signal to an input signal of the filter and expressed by the following equation (1):                               H          ⁡                      (            s            )                          =                                            V              ⁢              out                        ⁡                          (              s              )                                                          V              ⁢              in                        ⁡                          (              s              )                                                          (        1        )            
In Eq. (1), s=jxcfx89 where j represents an imaginary unit and xcfx89 represents an angular frequency.
The damping characteristic is expressed by the following equation (2):
20 log10|H(jxcfx89)|(dB)xe2x80x83xe2x80x83(2)
From Eq. (2), it is found that one-digit attenuation results in reduction by 20 dB (20 dB/dec).
The reason why the filter of FIG. 41 is referred to as an L-shaped primary filter is that the denominator or numerator of the transfer function of the filter is described by the linear function of s (=jxcfx89).
In the cases of filters of FIGS. 39 and 40, for example, the transfer functions HLPF (s) and HHPF (s) are expressed by the following equations (3) and (4), respectively:                                           H            LPF                    ⁡                      (            s            )                          =                                            1              sC                                      R              +                              1                sC                                              =                      1                          1              +              sCR                                                          (        3        )                                                      H            LPF                    ⁡                      (            s            )                          =                              R                          R              +                              1                sC                                              =                      sCR                          1              +              sCR                                                          (        4        )            
FIGS. 42 and 43 are schematic Bode diagrams of the LPF and the HPF. In FIGS. 42 and 43, the horizontal axis represents the frequency in logarithmic representation and the vertical axis represents the damping factor in logarithmic representation.
The frequency characteristic of the LPF shown in FIG. 42 indicates that the input signal is outputted without being attenuated in a low-frequency region and the input signal is attenuated and little outputted in a high-frequency region.
On the other hand, the frequency characteristic of the HPF shown in FIG. 43 indicates that the input signal is outputted without being attenuated in the high-frequency region and the input signal is attenuated and little outputted in the low-frequency region.
L-Shaped Secondary Filter
FIG. 44 shows an example of filter which is referred to as an L-shaped secondary filter.
In FIG. 44, the resistor R and an inductor L are interposed, being connected in series, between the terminals T1 and T3 and the capacitor C is interposed between the wire connecting the terminals T2 and T4 and an end portion of the inductor L on the side of the terminal T3.
The reason why the filter of FIG. 44 is referred to as a secondary filter is that the denominator or numerator of the transfer function of the filter is described by the quadratic function of s (=jxcfx89).
The filter of FIG. 44 is an LPF, and its transfer function H(s) is expressed by the following equation (5):                               H          ⁡                      (            s            )                          =                                            1              sC                                      R              +                                                                   s                                ⁢                L                            +                              1                sC                                              =                                    1                                                                    s                    2                                    ⁢                  LC                                +                sRC                +                1                                      =                                          ω                p                2                                                              s                  2                                +                                                                            ω                      p                                        Q                                    ⁢                  s                                +                                  ω                  p                  2                                                                                        (        5        )            
From the following equations (6) and (7) and the relation s=jxcfx89, the transfer function H(s) is transformed into the equation (8) as follows:                               ω          P                =                  1                                    L              ⁢                              xe2x80x83                            ⁢              C                                                          (        6        )                                Q        =                              1            R                    ⁢                                    L              C                                                          (        7        )                                          H          ⁡                      (            s            )                          =                              ω            P            2                                              ω              P              2                        -                          ω              2                        +                          j              ⁢                                                ω                  P                                Q                            ⁢              ω                                                          (        8        )            
From Eq. (8), it is found that the transfer function indicates the resonance characteristic when xcfx89=xcfx89p. The absolute value of the transfer function at that time is equal to Q-value (selectivity). In other words, it is preferable that the Q-value should be made as small as possible in order to suppress resonance.
FIG. 45 is a schematic view of the Bode diagram of the LPF shown in FIG. 44. FIG. 45, where the horizontal axis represents the angular frequency of Eq. (6) and the vertical axis represents the damping factor, shows the Bode diagram in the cases where the Q-value is 0.8, 2 and 10.
As shown in FIG. 45, it is found that the characteristic of the filter is distorted near the resonance frequency xcfx89p as the Q-value becomes larger.
The LPF of FIG. 44 is represented by using the impedances Z1, Z2 and Z3 as shown in FIG. 46, and it is possible to form an LPF and an HPF by changing the combinations of passive elements (resistor, capacitor, inductor) which are assigned to these impedances.
The transfer function of various secondary filters is expressed, in general, by the following equations (9), (10), (11) and (12):                               H          ⁡                      (            s            )                          =                  b                                    s              2                        +                          a              ⁢                              xe2x80x83                            ⁢              s                        +            b                                              (        9        )                                          H          ⁡                      (            s            )                          =                              s            2                                              s              2                        +                          a              ⁢                              xe2x80x83                            ⁢              s                        +            b                                              (        10        )                                          H          ⁡                      (            s            )                          =                  K          ⁢                                    f              ⁢                              xe2x80x83                            ⁢              s                                                      s                2                            +                              a                ⁢                                  xe2x80x83                                ⁢                s                            +              b                                                          (        11        )                                          H          ⁡                      (            s            )                          =                  K          ⁢                                                    s                2                            +              b                                                      s                2                            +                              a                ⁢                                  xe2x80x83                                ⁢                s                            +              b                                                          (        12        )            
Eqs. (9) and (10) represent the transfer functions of the LPF and the HPF, respectively, and Eqs. (11) and (12) represent the transfer functions of a band-pass filter (hereinafter, referred to as xe2x80x9cBPFxe2x80x9d) and a band-reject filter (hereinafter, referred to as xe2x80x9cBRFxe2x80x9d), respectively.
Another example of the L-shaped secondary filter is such a constitution as shown in FIG. 47 in which two L-shaped primary filters of FIG. 41 are connected to each other.
As shown in FIG. 47, the impedance Z1 is interposed between the terminals T1 and T3 and the impedance Z2 is interposed between the wire connecting the terminals T2 and T4 and an end portion of the impedance Z1 on the side of the terminal T3. Further, the impedance Z3 is interposed between the terminal T3 and a terminal T5 and the impedance Z4 is interposed between a wire connecting the terminal T4 and a terminal T6 and an end portion of the impedance Z3 on the side of the terminal T5. The terminals T1 and T2 serve as input terminals and the terminals T5 and T6 serve as output terminals.
The filter of FIG. 47 is also referred to as the L-shaped secondary filter since the denominator and numerator of its transfer function are described by the quadratic function.
When the passive elements are assigned so that the following relations should be satisfied, Z1=R1, Z2=1/sC2, Z3=R3 and Z4=1/sC4, for example, an LPF is formed. In this case, reference signs R1 and R3 represent resistance values, signs C2 and C4 represent capacitance values and s=jxcfx89.
Further, when the passive elements are assigned so that the following relations should be satisfied, Z1=1/sC1, Z2=R2, Z3=1/sC3 and Z4=R4, for example, an HPF is formed. In this case, reference signs R2 and R4 represent resistance values and signs C1 and C3 represent capacitance values.
Furthermore, when the passive elements are assigned so that the following relations should be satisfied, Z1=1/sC1, Z2=R2, Z3=1/sC3 and Z4=R4, for example, the ante-stage L-shaped filter forms an HPF and the post-stage L-shaped filter forms an LPF. The Bode diagram of this case is shown in FIG. 48.
In FIG. 48, the horizontal axis represents the frequency in logarithmic representation and the vertical axis represents the damping factor in logarithmic representation. As shown in FIG. 48, the input signal is outputted only in a certain frequency region. A filter having such a function is a band-pass filter (BPF).
Further, in FIG. 47, even when the passive elements are assigned so that the following relations should be satisfied, Z1=R2, Z2=1/sC1, Z3=1/sC4 and Z4=R3, a like BPF is achieved.
T-Shaped Bridge Secondary Filter
FIG. 49 shows an example of filter referred to as a T-shaped bridge secondary filter.
As shown in FIG. 49, the impedances Z1 and Z3 are interposed, being connected in series, between the terminals T1 and T3 and the impedance Z2 is interposed between the wire connecting the terminals T2 and T4 and a wire connecting the impedances Z1 and Z3. Further, an impedance Z4 is connected in parallel to the impedances Z1 and Z3 between the terminals T1 and T3.
In this constitution, when the passive elements are assigned so that the following relations should be satisfied, Z1=1/sC1, Z2=R2, Z3=1/sC3 and Z4=R4, for example, the impedances Z1, Z2 and Z3 form an HPF and the impedance Z4 forms an LPF.
Specifically, when the input signal is in the high-frequency region, the input signal is outputted through the HPF consisting of the impedances Z1, Z2 and Z3, and when the input signal in the low-frequency region, the input signal is outputted through the impedance Z4. In other words, the impedances Z1, Z2 and Z3 serve as the HPF and the impedance Z4 serves as the LPF. As a result, in a certain frequency region, no input signal is outputted. The Bode diagram of this case is shown in FIG. 50.
In FIG. 50, the horizontal axis represents the frequency in logarithmic representation and the vertical axis represents the damping factor in logarithmic representation. As shown in FIG. 50, no input signal is outputted only in a certain frequency region. A filter having such a function is a band-reject filter (BRF).
Further, even when the passive elements are assigned so that the following relations should be satisfied, Z1=R1, Z2=1/sC2, Z3=R3 and Z4=1/sC4, a BRF having like function is achieved.
Twin T-Shaped Bridge Secondary Filter
FIG. 51 shows an example of filter referred to as a twin T-shaped bridge secondary filter. As shown in FIG. 51, the impedances Z4 and Z6 are interposed, being connected in series, and the impedances Z1 and Z3 is interposed, being connected in series, between a terminal T10 serving as an input terminal and a terminal T20 serving as an output terminal. Further, an impedance Z5 is interposed between a wire connecting the impedances Z4 and Z6 and the ground potential, and the impedance Z2 is interposed between a wire connecting the impedances Z1 and Z3 and the ground potential.
In this constitution, when the passive elements are assigned so that the following relations should be satisfied, Z1=R1, Z2=1/sC2, Z3=R3, Z4=1/sC4, Z5=R5, Z6=1/sC6 and C1=C3=C5/2, R2=2R4=2R6, the filter shown in FIG. 51 serves as the BRF.
Secondary Active Filter (Sallen Key Type)
A filter including active elements such as a transistor, an op-amp (operational amplifier), a negative resistance element and a gyrator is referred to as an active filter. FIG. 52 shows an example of active filter referred to as a Sallen key secondary filter.
As shown in FIG. 52, the impedances Z1 and Z2 are interposed, being connected in series, between the terminal T10 serving as the input terminal and a noninverting input terminal of an op-amp OP, and an output terminal of the op-amp is connected to the terminal T20.
Further, the impedance Z3 is interposed between a wire connecting the impedance Z2 and the noninverting input terminal and the ground potential, and the impedance Z4 is interposed between a connecting node between the impedances Z1 and Z2 and the output terminal of the op-amp OP.
Furthermore, the resistors R2 and R1 are interposed, being connected in series, between the output terminal of the op-amp OP and the ground potential, and the connecting node between the resistors R2 and R1 is connected to an inverting input terminal of the op-amp OP.
In this constitution, when the passive elements are assigned in such combinations of the impedances Z1 to Z4 shown in the following table 1, an LPF, an HPF and a BPF can be achieved.
In the case of LPF, when the K-value gets closer to 3 in a case where the relation 1+R2/R1=K holds, it becomes difficult to adjust the Q-value since the Q-value becomes larger.
FIG. 53 shows the relation between the K-value and the Q-value in the case of LPF. As shown in FIG. 53, it is found that the Q-value increases without limit as the K-value gets closer to 3. Further, as discussed earlier, since the characteristic of the filter is distorted near the resonance frequency as the Q-value becomes larger, it is preferable to set the values of the resistors R1 and R2 so that the K-value should not get closer to 3.
Secondary Active Filter (Infinite Feed Back Type)
Another example of active filter is a constitution of an infinite feed back secondary filter shown in FIG. 54.
As shown in FIG. 53, the impedances Z1 and Z3 are interposed, being connected in series, between the terminal T10 serving as the input terminal and the inverting input terminal of the op-amp (operational amplifier) OP, and the output terminal of the op-amp OP is connected to the terminal T20. Further, the noninverting input terminal of the op-amp OP is connected to the ground potential.
Furthermore, the impedance Z2 is interposed between the connecting node between the impedances Z1 and Z3 and the output terminal of the op-amp OP, and the impedance Z4 is interposed between the connecting node between the impedances Z1 and Z3 and the ground potential.
The impedance Z5 is interposed between a wire connecting the impedance Z2 and the output terminal of the op-amp OP and a wire connecting the impedance Z3 and the inverting input terminal of the op-amp OP.
In this constitution, when the passive elements are assigned in such combinations of the impedances Z1 to Z5 shown in the following table 2, an LPF, an HPF and a BPF can be achieved.
Secondary Active Filter (Biquad)
As another example of active filter, a Tow-Thomas biquad circuit, which is a kind of Biquadratic circuit (abbreviated as xe2x80x9cBiquadxe2x80x9d) using three op-amps is shown in FIG. 55.
In FIG. 55, between the terminal T10 serving as the input terminal and the terminal T20 serving as the output terminal, op-amps OP1, OP2 and OP3 are connected in series.
Further, the resistor R1 is interposed between the terminal T10 and an inverting input terminal of the op-amp OP1, the resistor R2 is interposed between the output terminal of the op-amp OP1 and an inverting input terminal of the op-amp OP2 and the resistor R3 is interposed between the output terminal of the op-amp OP2 and an inverting input terminal of the op-amp OP3. Furthermore, noninverting input terminals of the op-amps OP1 to OP3 are connected to the ground potential.
A capacitor C1 and the resistor R4 are interposed, being connected in parallel, between the inverting input terminal and the output terminal of the op-amp OP1, a capacitor C2 is interposed between the inverting input terminal and the output terminal of the op-amp OP2, the resistor R5 is interposed between the inverting input terminal and the output terminal of the op-amp OP3, and the resistor R6 is interposed between the inverting input terminal of the op-amp OP1 and the output terminal of the op-amp OP3.
The characteristic feature of such a filter lies in higher Q-value, small element sensitivity, easy adjustment, realization of HPF output, LPF output and BPF output in a circuit and the like.
For example, the output terminal of the op-amp OP1 serves to make a BPF output in response to the input signal, and the output terminal of the op-amp OP2 serves to make an LPF output in response to the input signal.
Further, FIG. 56 shows a KHN Biquad circuit which is a kind of Biquad using three op-amps.
In FIG. 56, the op-amps OP1, OP2 and OP3 are connected in series between the terminal T10 serving as the input terminal and the terminal T20 serving as the output terminal.
The resistor R1 is interposed between the terminal T10 and the inverting input terminal of the op-amp OP1, the resistor R2 is interposed between the output terminal of the op-amp OP1 and the inverting input terminal of the op-amp OP2, and the resistor R3 is interposed between the output terminal of the op-amp OP2 and the inverting input terminal of the op-amp OP3. Further, the noninverting input terminals of the op-amps OP2 and OP3 are connected to the ground potential.
Furthermore, the resistor R4 is interposed between the inverting input terminal and the output terminal of the operational amplifier OP1, the capacitor C1 is interposed between the inverting input terminal and the output terminal of the op-amp OP2, the capacitor C2 is interposed between the inverting input terminal and the output terminal of the op-amp OP3, the resistor R5 is interposed between the noninverting input terminal of the op-amp OP1 and the output terminal of the op-amp OP2, and the resistor R6 is interposed between the inverting input terminal of the op-amp OP1 and the output terminal of the op-amp OP3.
The KHN Biquad circuit is named from the capitals of Kerwin, Huelsman and Newcomb, in which the output terminal of the op-amp OP1 serves to make an HPF output in response to the input signal, the output terminal of the op-amp OP2 serves to make a BPF output in response to the input signal, and the output terminal of the op-amp OP3 serves to make an LPF output in response to the input signal.
As discussed above, the filter uses such resistance elements, and the conventional resistance elements use metal, doped polysilicon, switch capacitor, OTA (Operational transconductance amplifier) and the like.
Whatever is used as the material of the resistance elements, however, since variations in resistance value are inevitably caused by process variations in a manufacturing process, there is a problem that even the same filters should have variations in frequency characteristics.
Further, though a variable resistance may be useful in order to suppress the variations in frequency characteristics among the filters, since it is difficult to form a small-sized variable resistance, the idea is not actually realized. This also applies to semiconductor integrated circuits other than filters.
It is an object of the present invention is to provide an analog circuit including filters, amplifiers or the like, which can correct the variations in value of elements caused by the process variations in the manufacturing process.
The present invention is directed to a semiconductor memory device. According to the present invention, the semiconductor memory device at least includes an analog circuit having a variable resistance unit which consists of a plurality of magnetic tunnel resistance elements and obtains a plurality of kinds of resistance values by using the plurality of magnetic tunnel resistance elements singly or in combination and changing the resistance values of the plurality of magnetic tunnel resistance elements by single ones or combinations, being capable of changing the resistance values of the plurality of magnetic tunnel resistance elements by single ones or combinations with a plurality of control signals, a control unit for outputting the plurality of control signals, and a memory cell array. The semiconductor memory device adopts address signal multiplexing in which an address terminal is properly used in a time-division manner. The control unit uses an address decoder of the memory cell array also as a decoder for resistance-value control of at least one of the plurality of magnetic tunnel resistance elements. The address decoder is controlled on the basis of a magnetic tunnel resistance element control signal which is given to the address terminal in a time-division manner, for the resistance-value control of the at least one magnetic tunnel resistance element.
Since the analog circuit includes the variable resistance unit capable of obtaining a plurality of kinds of resistance values, it is possible to control circuit characteristics by changing the resistance values of the variable resistance unit. Further, since the address decoder of the memory cell array is used also as a decoder for resistance-value control of the magnetic tunnel resistance element and the address decoder is controlled on the basis of the magnetic tunnel resistance element control signal, it is possible to suppress upsizing of a semiconductor memory device incorporating the analog circuit using the magnetic tunnel resistance.